Because the process of shaving places the blade steel in contact with moisture, stainless steel is a natural selection for razor blade applications. Razor blades typically are fabricated from a coil of stainless steel that has been rolled to a strip of very thin gage (less than ten mils) and that has been slit to an appropriate width. The coiled steel strip is uncoiled, sharpened, hardened, appropriately coated, and welded to a blade support so that it may be manipulated against the skin.
Steel used as razor blade material preferably includes secondary carbide particles that are of a uniform generally spherical shape, that have uniform size less than 15 micrometers and uniform distribution, and that are present in a concentration of about 50-200 carbide particles per 100 micrometers square as observed at high magnification. If secondary carbide particles within the steel are not of uniform size and distribution, for example, the steel may distort during the heat treatments used in razor blade fabrication. Distortion of the steel during heat treatment is referred to as "dish", and only a minor amount of dish is cause for rejecting the steel. The steel preferably also is substantially free of primary carbides or clusters of carbides that exceed 15 micrometers in length. It is also preferred that the steel is essentially free of non-metallic microinclusions and does not include regions of segregation, carburization, or decarburization. Primary carbide particles and non-metallic microinclusions typically are large in size, brittle in nature, and have a low cohesion to the steel matrix. As such, they may cause "tear outs" during the sharpening of the steel. A tear out occurs during sharpening when the carbide particle or inclusion is pulled from the steel, leaving a jagged surface that can be felt during shaving.
In addition to meeting the foregoing microstructural criteria, stainless steels used in razor blade fabrication also must satisfy additional qualitative and quantitative criteria established by the individual razor blade manufacturers and which demonstrate a suitability for shaving. Certain of those additional criteria are evaluated after samples of the steel strip have been modified by the manufacturer to include a sharpened edge, additional martensite (i.e., enhanced hardness), and a non-metallic coating.
Razor blades are commonly fabricated from strip of certain high carbon type 420 stainless steels. (Type 420 steels have the nominal composition 0.15 min. carbon, 1.00 max. manganese, 1.00 max. silicon, and 12.0-14.0 chromium, all in weight percent.) The type 420 steels that may be used as razor blade material must have a chemistry that may be processed to meet the above microstructural requirements. The steels also must be capable of processing to a uniform thin gage strip, typically 34 mils in thickness, a uniform width, and have no appreciable surface defects or edge checking. Because the steel strip typically is produced from large ingots weighing thousands of pounds, the overall thickness reduction necessary to achieve 3-4 mils thickness during processing is extreme. The need to achieve a thin gage final material while also meeting the other requirements discussed above necessarily complicates the processing of the material and limits the array of suitable heat chemistries and processing regimens.
Accordingly, there is a need for a method of processing type 420 and other stainless steels to a uniform thin gage while satisfying the above microstructural criteria. There also is a need for improved martensitic stainless steel alloys that demonstrate a suitability for razor blade applications.